This invention relates generally to offset lithographic machines, and more particularly to a device for cleaning a blanket cylinder in an offset lithographic press and the like.
In the offset lithographic press (or offset printing press) widely used in recent years, there are ordinarily provided three cylinders, namely, a plate cylinder on which a metal printing plate is clamped, a blanket cylinder wrapped with a rubber blanket, and an impression cylinder urging a sheet stock to be printed against the blanket cylinder. Ink is supplied from an ink supplying device, and extended over the printing plate so that the ink is deposited on the printing part of the printing plate owing to, for instance, affinity of that part to the ink. Simultaneously therewith, water is supplied to dampen the nonprinting plate.
In the course of the rotations of the three cylinders, a major part of the ink deposited on the printing part of the printing plate is transferred from the surface of the plate cylinder to the corresponding part of the blanket cylinder and then transferred (offset) therefrom to the surface of the sheet stock, such as paper, which is fed through a paper guide plate provided in the neighborhood of the impression cylinder, and caught by teeth provided around the lateral ends of the impression cylinder.
In an offset lithographic press of the above described character, wherein the ink deposited on the plate cylinder is once transferred onto the blanket cylinder and then retransferred onto the paper pressed against the blanket cylinder by the impression cylinder, it is essential that any dirt or dust, inclusive of deposition of ink, collected on the surface of the blanket cylinder be cleaned off thoroughly.
Heretofore, such a cleaning operation has been carried out by hand while the cylinders are rotated at low speeds, by pressing, for instance, a piece of cloth impregnated with a cleaning liquid against the surface of the blanket cylinder. Such an operation, however, is not only a labor consuming work, but also is extremely dangerous because of the possibility of the operator's hand being caught between the rotating cylinders.
In order to overcome the above mentioned disadvantages of the manual method of cleaning the blanket cylinder, there has been proposed an automatic cleaning device for the blanket cylinder, which comprises an endless belt extended around a driving wheel and a dummy wheel so that the endless belt is driven in a direction parallel to the rotating axis of the blanket cylinder, a layer of sponge secured onto the outer surface of the endless belt in a manner such that the layer can be brought into contact with the outer surface of the blanket cylinder when the driving wheel and the dummy wheel are both shifted toward the blanket cylinder, a squeeze roller forced onto the sponge layer on the outer span of the endless belt, and a nozzle provided at an upstream position relative to the squeeze roller for supplying a cleaning liquid onto the sponge layer on the endless belt.
With the automatic cleaning device of the above described construction, the outer surface of the blanket cylinder is cleaned, while it is rotated at a low speed, by the sponge layer on the endless belt impregnated with the cleaning liquid, and the used dirty liquid is squeezed out of the sponge layer by the squeeze roller during the travel of the endless belt.
However, the automatic cleaning device of the above described construction inevitably becomes of a greater size, and consumes a comparatively greater quantity of cleaning liquid. Furthermore, the device cannot remove any oxide layer which might be created on the outer surface of the blanket cylinder, and the working life of the endless belt also has been relatively short.